32. The soils of this class that are met with in preparing the foundations of buildings are generally along the shore of some large body of water, and hence generally permeated with water to within a few feet of the surface.
For such soils pile foundations are usually the cheapest and most reliable. On a soil such as underlies Chicago, and having a supporting power of from 1½ to 2½ tons per square foot, spread foundations may be used with satisfactory results and with economy, when it would require piles over 40 feet long to reach hard pan.
Occasionally it is necessary to build on ground that has been filled in to a considerable depth, and in which water is not present, or the building may be so heavy that it is impracticable to support it on piles: in such cases wells of solid masonry, with an iron casing, or pneumatic caissons, should be sunk to bed rock or hard pan, as hereinafter described.